Looking young in old age

Can one’s gray hair spontaneously revert back to its original color?

A religious woman and a female soldier pray at the Western Wall..
(photo credit:MARC ISRAEL SELLEM/THE JERUSALEM POST)

My husband is 82 years old and his very dark hair started graying in his early 30s.By the time he was in his 50s, it was already completely white. His hair has thinned out, but it still covers his whole head. Strangely, over the last few months, his hair is growing out black. Can you explain this phenomenon? – R.S.,